The ability to modify the threshold voltage in a transistor is highly desirable to design complex circuits. In Silicon MOSFETs, threshold voltage is commonly tuned by the doping density in the channel. However, the change in threshold voltage in unipolar transistors is not easy. For example, the threshold voltage of unipolar n-type transistors of III-V and III-Nitride semiconductors is typically negative. Therefore these transistors are depletion-mode or normally-on devices. Although enhancement-mode or normally-off transistors are highly desirable in many applications, it can be difficult to change the threshold voltage of these transistors to positive values.
This invention describes a new structure with island regions in the gate region of a transistor. This new structure is useful for tuning the threshold voltage.